How to Use D-Noise in the Compositor?
I think I've worked it out: in the Compositor, remove the Render Layers node and add an Image Node and a Denoise Node like this:



Now open the first image in your sequence, tell the Image Node that this is indeed an Image Seqeunce and choose the number of frames it has.



Now you can render as usual and see the result, either as a single image or as an animation. Specify the number of frames to be rendered and your output path as if this was a 3D scene.

Note that the results of applying D-NOISE this way are far inferior to applying it during a render. I guess OptiX has a lot more information to work with when the 3D scene is loaded, as opposed to seeing only the 2D image. Here's a comparison between D-NOISE when a applied with a render, and after the fact. Notice the loss of detail in the second image.





In regards to D-NOISE not kicking in: try starting Blender as an Administrator (if it's a Windows system). I've had the same issue with some of my nodes, when run as Administrator D-NOISE behaves fine.

Hope this helps!